bug#63518: 28.2; shr.el seems to break inline latex (mathjax) in html

[mousebot]

hi emacs,

The fediverse client I maintain, mastodon.el, uses shr-render-region to render 
individual posts. Some instances, e.g. https://mathstodon.xyz, allow users to 
post inline latex using mathjax notation.

When shr.el renders inline latex, it often breaks it as it fills the text. It 
inserts a newline in between the two characters that open an inline latex 
block: `\(` or `\[`. Using normal fill commands to fill text (fill-region, fill 
paragraph) do not split latex in this way, from what I could gather.

When digging around and debugging a little, I found that in 
shr-find-fill-point, the check (shr-char-kinsoku-eol-p (following-char)) in the 
when condition returns t when point is in between \ and ( or [, meaning that 
shr-find-fill-point considers that position to be a breakable point. Commenting 
that single check seems to largely prevent the undesired splitting. (Behaviour 
confirmed by my checks and also by another mastodon.el user.)

I don't really understand the significance of the checks that 
shr-find-fill-point runs, nor whether they can be temporarily deactivated or 
worked around in some other way.

I read around a little, and asked on emacs.stackexchange, but received no 
replies. So I'm still unsure if shr is able to handle mathjax notation or not, 
or how to patch it so that it would respect it.

An example thread containing inline latex: 
https://mathstodon.xyz/@bones/110249960030484103.

An example of the html that may break if window width is close to a latex 
snippet (from the above thread):

<p>Apéry’s proof utilized two surprising sequences of numbers \\(A_n\\) and 
\\(B_n\\), which satisfy the recurrence relation \\[(n+1)^3 x_{n+1}-\\left(34 n^3+51 n^2+27 
n+5\\right) x_n+n^3 x_{n-1}=0 \\]  with initial conditions \\((A_0,A_1)=(1,5)\\), 
\\(B_0,B_1)=(0,6)\\). Apéry showed that \\(A_n \\in \\mathbb{Z}\\) for all \\(n \\geq 0 
\\), which is quite surprising! You can check that the first few numbers in the sequences 
are given by \\[A_n = 1, 5, 73, 1445, \\dots \\] \\[B_n = 0, 6, \\frac{351}{4}, \\dots \\]  
The result of Apéry is that the sequence \\(B_n/A_n \\to \\zeta(3)\\) sufficiently fast to 
guarantee that \\(\\zeta(3)\\) is irrational, by Dirichlet’s irrationality 
criterion.</p>

This is my first time reporting a bug in emacs, apologies if there's anything 
wrong the report. I'm happy to provide further details if needed.

Thanks,
Marty.

original report on mastodon.el repo: 
https://codeberg.org/martianh/mastodon.el/issues/464

stack exchange question: 
https://emacs.stackexchange.com/questions/77214/shr-filling-dont-split-inline-latex-in-html

report-emacs-bug details from a minimal emacs:

In GNU Emacs 28.2 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, cairo 
version 1.16.0)
 of 2023-01-21 built on t470s
Windowing system distributor 'The X.Org Foundation', version 11.0.12011000
System Description: Debian GNU/Linux 11 (bullseye)

Configured using:
 'configure --prefix=/home/mouse/programmes/emacs-28.2/
 --bindir=/home/mouse/bin'

Configured features:
CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LIBOTF LIBSELINUX LIBXML2 M17N_FLT MODULES NOTIFY INOTIFY PDUMPER PNG
RSVG SECCOMP SOUND THREADS TIFF TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM
GTK3 ZLIB

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Mastodon

Minor modes in effect:
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  cua-mode: t
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  emojify-mode: t
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  buffer-read-only: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t

Load-path shadows:
/home/mouse/code/elisp/mastodon.el/lisp/mastodon-iso hides 
/home/mouse/.emacs.d/straight/build/mastodon/mastodon-iso
/home/mouse/code/elisp/mastodon.el/lisp/mastodon-search hides 
/home/mouse/.emacs.d/straight/build/mastodon/mastodon-search
/home/mouse/code/elisp/mastodon.el/lisp/mastodon-async hides 
/home/mouse/.emacs.d/straight/build/mastodon/mastodon-async
/home/mouse/code/elisp/mastodon.el/lisp/mastodon-http hides 
/home/mouse/.emacs.d/straight/build/mastodon/mastodon-http
/home/mouse/code/elisp/mastodon.el/lisp/mastodon hides 
/home/mouse/.emacs.d/straight/build/mastodon/mastodon
/home/mouse/code/elisp/mastodon.el/lisp/mastodon-media hides 
/home/mouse/.emacs.d/straight/build/mastodon/mastodon-media
/home/mouse/code/elisp/mastodon.el/lisp/mastodon-discover hides 
/home/mouse/.emacs.d/straight/build/mastodon/mastodon-discover
/home/mouse/code/elisp/mastodon.el/lisp/mastodon-client hides 
/home/mouse/.emacs.d/straight/build/mastodon/mastodon-client
/home/mouse/code/elisp/mastodon.el/lisp/mastodon-auth hides 
/home/mouse/.emacs.d/straight/build/mastodon/mastodon-auth
/home/mouse/code/elisp/mastodon.el/lisp/mastodon-notifications hides 
/home/mouse/.emacs.d/straight/build/mastodon/mastodon-notifications
/home/mouse/code/elisp/mastodon.el/lisp/mastodon-profile hides 
/home/mouse/.emacs.d/straight/build/mastodon/mastodon-profile
/home/mouse/code/elisp/mastodon.el/lisp/mastodon-views hides 
/home/mouse/.emacs.d/straight/build/mastodon/mastodon-views
/home/mouse/code/elisp/mastodon.el/lisp/mastodon-tl hides 
/home/mouse/.emacs.d/straight/build/mastodon/mastodon-tl
/home/mouse/code/elisp/mastodon.el/lisp/mastodon-toot hides 
/home/mouse/.emacs.d/straight/build/mastodon/mastodon-toot
/home/mouse/code/elisp/mastodon.el/lisp/mastodon-inspect hides 
/home/mouse/.emacs.d/straight/build/mastodon/mastodon-inspect

Features:
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button loaddefs faces cus-face macroexp files window text-properties
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Memory information:
((conses 16 311432 41501)
 (symbols 48 25123 6)
 (strings 32 147666 7861)
 (string-bytes 1 6525722)
 (vectors 16 73699)
 (vector-slots 8 3025398 126294)
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 (intervals 56 1691 262)
 (buffers 992 15))

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